Certainvirusesandtoxinscanbeconveyedbyaxonaltransportalongperipheralnerves.Forexample,herpeszoster,thevirusofchickenpox,invadesdorsalrootganglioncells.Thevirusmaybeharboredbytheseneuronsformanyyears.However,eventuallythevirusmaybecomeactivebecauseofachangeinimmunestatus.Thevirusmaythenbetransportedalongthesensoryaxonstotheskin,causingshingles,averypainfuldisease.Anotherexampleistheaxonaltransportoftetanustoxin.Clostridium tetani bacteriamaygrowinadirtywound,andifthepersonhadnotbeenvaccinatedagainsttetanustoxin,thetoxincanbetransportedretrogradelyintheaxonsofmotorneurons.Thetoxincanescapeintotheextracellularspaceofthespinalcordventralhornandblockthesynapticreceptorsforinhibitoryaminoacids.Thisprocesscanresultintetanicconvulsions.
The major nonneuronal cellular elements of the nervous system are the glia (
Fig. 4.4 ). Glial cells in the human CNS outnumber neurons by an order of magnitude: there are about 1013 glia and 1012 neurons. Glial cells in the CNS include astrocytes, oligodendrocytes, microglia, and ependymal cells (see
Fig. 4.4 ); in the PNS the glial cells are Schwann cells and satellite cells. Traditionally, glial cells were thought of as supportive cells, and consistent with that conception, their functions include regulation of the microenvironment and myelination of axons. Glial cells are now also recognized to be important determinants of the flow of signals through neuronal circuits, and related to that they act to modulate synaptic and nonsynaptic transmission and have important roles in synaptogenesis and maintenance.
Astrocytes (named for their star shape) help regulate the microenvironment of the CNS, both under normal conditions and in response to damage to the nervous system. Astrocytes have a cell body from which several main branches arise. Through repeated branching these main processes give rise to hundreds to thousands of branchlets. Astrocyte processes contact neurons and surround synaptic endings, isolating them from adjacent synapses and the general extracellular space. Astrocytes also have foot •Fig. 4.4 SchematicrepresentationofcellularelementsintheCNS.Twoastrocytesareshownendingonasomaanddendritesofaneuron.Astrocytesalsocontactthepialsurfaceorcapillariesorboth.Anoligodendrocyteprovidesthemyelinsheathsforaxons.Alsoshownaremicrogliaandependymalcells. N,neuron.(RedrawnfromWilliamsPL,WarwickR.Functional Neuroanatomy of Man. Edinburgh:ChurchillLivingstone;1975.) processes that contact the capillaries and connective tissue at the surface of the CNS, the pia mater (see
Fig. 4.4 ). These foot processes may help mediate the entry of substances into the CNS. Astrocytes can actively take up K+ ions and neurotransmitter substances, which they metabolize, biodegrade, or recycle. Thus astrocytes serve to buffer the extracellular environment of neurons with respect to both ions and neurotransmitters. The cytoplasm of astrocytes contains glial filaments that provide mechanical support for CNS tissue. After injury the astrocytes undergo a variety of changes to become reactive astrocytes. One example is a class of reactive astrocytes that act to form a glial scar around an area of focal damage, which segregates the damaged tissue and thereby allows inflammatory processes to act selectively at the site of damage, minimizing the impact on surrounding normal tissue. Astrocytes can also affect the properties of synaptic transmission, which is discussed in